Structural parts consisting of noble metal and noble metal alloys, such as preferably PGM material (PGM=Platinum Group Metals), are used in the glass industry, particularly in plants for melting and hot forming of special glass. These plant components used in melt technology, also called PGM products, serve for melting, refining, transporting, homogenizing and apportioning the liquid glass.
Such structural parts are essentially either structures consisting of solid PGM material or of materials resistant to high temperature (ceramic refractory materials, metallic special materials) with thin-walled, protective PGM cladding, for example in the form of thin sheet metal or a PGM surface coating (applied for example by plasma spraying or flame spraying, etc.).
Plant parts carrying glass melts are often sheet structures of noble metals which are designed as thin-walled pipe systems. The molten glass flows through these at temperatures of between 1000° C. and 1700° C.
On account of their high melting point, PGM materials are distinguished by high temperature resistance and, furthermore, by their high mechanical strength and resistance to abrasion, and are therefore especially suitable for the production of structural parts in plants or plant parts which come into contact with the glass melts. Suitable materials are platinum and alloys of platinum and/or other metals of the platinum group, which may optionally also contain minor amounts of base metals as further alloying components or oxidic additives. Typical materials are refined platinum, platinum-rhodium alloys and platinum-iridium alloys, which contain a small amount of finely distributed refractory metal oxide, such as in particular zirconium dioxide or yttrium oxide, to increase the strength and high-temperature creep resistance.
The glass melting process breaks down into the following phases: melting, refining, conditioning, feeding and forming. To increase the degree of homogenization of the glasses, stirrers are used. Stirring is part of conditioning and consequently takes place after refining and before feeding. The variation in viscosity of the glass with temperature is of fundamental significance for all glass technology. To achieve a homogeneous melt, it must be brought to a temperature at which the dynamic viscosity is η˜102 dPa·s. For comparison: at 20° C., water has a viscosity of 0.01 dPa·s, olive oil about 102 dPa·s and honey about 104 dPa·s. Hot processing, that is feeding and forming of glass, is performed at 103 to 108 dPa·s, depending on the process. Consequently, the viscosity of the glass during stirring lies between 102 and 104 dPa·s. At ˜1450° C., the dynamic viscosity of borosilicate glass, for example, is η103 dPa·s.
As the cited data, temperature and dynamic viscosity show, the effective stirring of glass presents a technical challenge.
Stirring is among the most important basic process engineering operations. In its simple form, two or more components are united with one another and distributed within one another by introducing flow movements with the aid of the stirring tool in such a way that a uniform composition in the smallest possible units of volume is obtained.
The following 4 stirring tasks can be defined: homogenization, suspension, dispersion and heat transfer.
Heat transfer, the exchange of heat between the material being mixed and the surrounding medium through the wall of the mixer possibly takes place, but plays a minor role in the design of the stirring systems for glass.
Since, in the case of glass, the main phase and the additional phase are liquids, the stirring task is exclusively that of homogenization. Homogenization is the mixing of solids or liquids that are soluble in one another as well as the equalizing of differences in concentration and/or differences in temperature.
Mixing in turn means in principle the transporting of components of a material being mixed. In this case it is possible to distinguish between 5 individual basic operations, which under some circumstances can lead from one to the other:
Distributive mixing: distributing, blending, particle interchange on the basis of an ordering matrix and random matrix. In physical terms, gravitational force and Coulomb friction have to be overcome.
Dispersive mixing: breaking down aggregates and agglomerates. In this case, the resistance caused by adhesive stresses has to be overcome.
Laminar mixing: stretching, compressing, folding and overcoming Newtonian friction.
Turbulent mixing: creation of turbulent flow(s) in liquids and gases.
Diffuse mixing: concentration equalization by diffusion. Example: fluids at rest.
In case of mixing the highly viscous substance glass, this consequently involves laminar mixing and distributive mixing, this operation being very closely akin to kneading.
Kneading means the mixing of pasty substances of high viscosity. The energy input involved is many times higher than when mixing substances of low viscosity. If the working process of ‘kneading’ is considered from the point of view of flow behavior, the absence of turbulence may be mentioned as characteristic of the intensity of the mixing operation. The mass transfer takes place by shearing, mechanical division and compression.
The difficulty when processing liquids of high viscosity is the laminar flow behavior. For any mixing process, this behavior means that there are problems in the exchange of the corresponding flow filaments and components to be mixed. In the case of laminar flow, the forces caused by the viscosity (shearing stress, shearing) are dominant.
In order to achieve a defined mixing result, it is a prerequisite that the laminar flow affects the entire volume of the vessel.
In cases of high viscosities, as are usual with glass, only forced feeding can ensure homogenization of adequate quality.
In the prior art of the glass industry, the plant component that is the stirrer undertakes the homogenization of the glass melts in a crucible or stirring part or stirring cell. The stirring vessels always have a cylindrical or slightly conical form with “smooth” walls. In the continuous melting process, the glass is fed to the stirring vessel laterally from the top or the bottom through an inlet pipe. The glass then leaves laterally at a different height than the inlet by means of an outlet pipe or through the bottom of the vessel. The difference in height between the inlet and the outlet makes it possible in a continuous glass melting process to dispense with the force-feeding effect of the stirring element, since the entire volume of glass must pass through the stirring vessel. Consequently, the stirring task is that of laminar and distributive mixing that affects the entire volume of the vessel.
DE 10 2004 034 798 A1 relates to a stirring system for glass melts. In this case, a stirrer with a shaft, which defines a longitudinal axis, and at least two groups of paddles is provided. The paddles respectively comprise a blade, which is aligned parallel to the longitudinal axis, and at least one opening. At least two groups of paddles are positioned on the shaft at a distance from one another. Also provided is at least one group of baffles, which are positioned between two groups of paddles.